Microfluidic devices are devices which are capable of handling small amounts of chemical, bio-chemical or biological substances, i.e. for the analysis thereof. Microfluidic devices may comprise microfluidic channels, valves and other structures, including sensors and electronic circuitry to operate. Complex structures can be built on for example semiconductor components having dimensions in the order of micrometers.
Microfluidic devices can be built in a two-part form having a micromachined substrate and a microfluidic component mechanically, fluidically and electrically connected to the substrate. The substrate usually comprises a micromachined channel plate. The microfluidic component usually comprises a micromachined fluidic chip. A common method of mounting the microfluidic component on the substrate is called Flip-chip technology. In Flip-chip technology mechanical, microfluidic and electrical structures present in the substrate and microfluidic component can be connected by mutually corresponding connections in the surfaces of the respective parts facing each other. Such connections include corresponding access ports of microfluidic channels which run through the substrate and extend in the microfluidic component, and mechanical and electrical connections.
Microfluidic devices can be used beneficially in high temperature applications such as gas chromatography, where robustness of the fluidic and electrical connections when subjected to temperature variations plays a key role. In such applications, the fluidic connections should normally be gas tight, typically up to 5 bar with no or very low leak rates, and the electrical connections should be low ohmic. The temperature range over which the assembly should stay intact is typically −20 to +200 C.
In order to make the mechanical and fluidic connection as described, the microfluidic component and substrate can be connected using an adhesive layer. An adhesive layer can be formed by using a preformed layer sandwiched between the substrate and microfluidic component, or by applying an adhesive to mechanical structures designated for mechanically connecting the parts together. The electrical connection can be made by using conductive bumps for example gold bumps which are sandwiched between corresponding contact pads between the two facing surfaces. The conductive bumps electrically bond the respective contact pads when the microfluidic component is mounted on the substrate.
Microfluidic devices generally may have dimensions in the order of 3-15 mm, however larger or smaller dimensions may apply. Electrical connections in microfluidic devices can be normally sized in a range of 50-300 micrometer, whereas microfluidic access ports can be sized in a range of 50-1500 micrometer. With such small dimensions, microfluidic access ports and their associated channels acts as capillaries. Adhesively connecting the microfluidic component to the substrate with structures having such small dimensions requires the application of adhesive to be patterned and accurately aligned between the substrate and microfluidic component. Misalignment and excess adhesive may cause an overflow of adhesive from the mechanical connecting structures to functional parts of the substrate and/or microfluidic components due to their capillary action, thereby adversely affecting their function. One way to solve this is by applying adhesive in the form of a patterned adhesive preform. However, this requires an additional component, i.e. the preform, which also requires accurate patterning, positioning and aligning. Moreover, creating an adhesive bond in this manner requires exerting a considerable pressure to the microfluidic components and substrate, which may result in mechanical stress or even damage to either of the microfluidic parts. A further disadvantage is that air may become trapped between preform and component surfaces during assembly, resulting in poor adhesion properties. In the art gaskets have been used for sealing off microfluidic channels and preventing sealant, i.e. adhesive to spill into these channels and ports, impairing the microfluidic function and integrity. The use of gaskets also requires separate components, i.e. the gaskets, which also require positioning and aligning. Moreover, such gaskets require mechanical stress to perform the required sealing.
Furthermore, in the art, as described for example in U.S. Pat. No. 8,916,111, adhesive is applied in cavities between a substrate and a microfluidic component as an underfill for providing additional bonding strength between these parts. This solution however is not compatible with the required robustness with respect to temperature variations. Differences between thermal expansion coefficients between the adhesive used for this purpose and the material of the substrate may cause mechanical tension between the substrate and the microfluidic component and cause subsequent release of the bond and/or leaking of microfluidic structures within the substrate or microfluidic component. Also air bubbles trapped in the relatively thick adhesive layer, i.e. underfill, within the cavities may expand and cause breaking of the bond between substrate and microfluidic component bonded to the substrate during thermal cycling. This is sometimes referred to as popcorn effect. Delaminarion or peel-off of the microfluidic component starts off with a local release which is then propagated throughout a larger part of the adhesive layer between the substrate surface and microfluidic component.
In case of a combination of fluidic and electrical connections, thermal stress will occur since materials used in contact bumps for electrical connection, such as gold, and silicon have different thermal expansion coefficients. In general, there is a risk is that the electrical connection will be lost due to too high stress in the gold bumps.